Download - Instantaneous Packet Delay Variation
Instantaneous Packet Delay Variationipdv
Carlo M. Demichelis - CSELT
41th IETF - Los Angeles
1) INTRODUCTION
About definitions
One or Two point measurements
Reference to a given Transfer Delay
Reference to the mean value of Delay
Instantaneous Packet Delay Variation
- Definition
- Distributions
2) MEASUREMENTSimulated traffic and tools
Layout of a network measurement
Results with synchronized clocks
Results with Not synchronized clocks
3) FURTHER POINTS FOR STUDY
About definitions
Being a Variation, some different definitions are possible, according to whatthe variation is measured against. Four possibilities have been considered.
1) - Variation of Inter Arrival time with respect an expexted onesimilar to the 1-point CDV defined for ATM cells (ITU-T Rec. I.356)
2) - Delay Variation referred to a reference value for those Sec and Dstsimilar to the 2-point CVD defined for ATM cells (ITU-T Rec. I.356)
3) - Delay Variation with respect a mean value of the delay
4) - Delay Variation with respect the last preceding packet (ipdv)
One or Two point measurements
The 1-point measurement includes the behavior of the source and thenetwork. It has been considered more appropriate for characterizing a traffic profileat a given point than to analyze the behavior of a path between two points.
Even if 1-point measurements are performed at the same time at thebeginning and at the end of a path, and then the results are compared, it is difficultto derive the behavior of the path from statistics.
The choice was to focus on a 2-point PDV measurement, that seems the morenatural way for obtaining useful information on the behavior of the path.
At the same time the need was considered of having a metric that describedthose variations in dealy affecting real-time services, and as much as possibleindependent from synchronization problems.
Reference to a given Transfer Delay
For a given <Src, Dst, path> triad a Reference Transfer Delay “R” is given and the PDV value of the packet “n” having a transfer delay of “Dn” is given by:
PDVn = Dn - R
Some problems with this definition:
1) - It does not contain a strong idea ofvariation, being a Delay minus afixed offset
2) - It does not describe the time in whichthe variation takes place
3) - If the Delay is different from R but itdoes not change, values of PDVare anyway high
4) - A Reference Delay “R” should be defined for each possible path
5) - The distribution of values is distorted by changing conditionsof the path
Min. Delay
Ref. Delay “R”
Delay
%
PDV
%
Errors depend on duration of the measurement
Reference to the Mean Value of Delay
Transmission time
pdv1 (+)
pdv2 (-)
pdv3 (-)
pdv4 (+)
pdv5 (+)
pdv6 (0)pdv7 (+)Reception time
Each circle indicates a packet as characterized by aTx time and a Rx time.
The red straight line can be computed as the oneminimizing the vertical distances between circles andline
If there exist an offset between clocks, line and pointsare traslated up or down, the distances are unchanged
If there exist a skew between clocks the slope of thestraight line is higher or lower than 1.
Rec. Time
Transm. Time
“Local” Mean Value
Approx. Mean Value
Uncertainty strip
The method is derived from the preceding definition, and is away of chosing the best refernce value.
A straight line can be a not good approximation in case ofvariation of the mean value along the time
Also in this case the strip of uncertainty can increase inwidth according to the duration of the measuremet
Instantaneous Packet Delay Variation (1)Definition
For a given stream of packets, flowing from a Src to a Dsr the Instantaneous Packet DelayVariation (ipdv) of a packet is the difference between the Transfer Delay of the packet andthe Transfer Delay of the preceeding packet.If D(i) is the Transfer Delay of the i-th packet, its ipdv (ipdv(i)) is:
ipdv(i) = D(i) - D(i - 1)
The ipdv(0) of the first packet is not defined
The Measurement Points MP1 at Src and MP2 at Dst are virtually located at the first and, respectively,last point of the intreface with the transport network, at which it is (virtually) possible to observe the packets at wire-time.Practical measurement points will be the first and the last ones at which it is possible to deal with time-stamps (host-time). For referring the measurement to MP1 and MP2 it is necessary to know the diffe-rence (host-time vs wire-time) related to each packet. Otherwise that difference will be part of the measurement error.
According to its definition, ipdv is only affecred by the variable part of this difference.
Instantaneous Packet Delay Variation (2)Distributions
p(ipdv)
ipdv
ipdv
Probability (frequency) distribution- The theoretical mead value should tend to ZERO- Number of values exceeding given thresholds indicate how can be supported a given service- Standard Deviation is an indicator of the Quality- For better describing the behavior, SD can be also calculated over the sub-set of values inside the threshold limits.
Cumulative curve- Evidentiates the performance parameter “Percentile”- Evidentiates the performance parameter “Inverse Percentile”
In principle, results are not affected by the duration of the measurement
p(ipdv)
Instantaneous Packet Delay Variation (3)The Mean Value
Reasnons that can produce a non ZERO value
1) - Effect “Last Value” in short measurementsE(ipdv) = [D(last) - D(first)] / N
2) - Monotone variation of Average Delay inside limited periods
3) - Skew of the clocks (fixed)
4) - Drift of the clocks (limited by a max. skew that can be reached)
2) to 4) variation effects are limited to IAT ofthe packets.
0 Mean Value
For example (typical):
120000 packetsAverage of intervals = 0.75 sec
skew -> 50 µsother -> 5 µs
Measurement with simulated traffic
Src T = .5 to 1.5 secHost -Wireup to 100 µs
Network
Delay from 100 ms to 500 msplus
Additional Delay due to traffic variationup to 300 ms in 4 hours and down
Wirw-Hostup to 100 µs
Sync
No sync
Dst
Dst
loop
loop
Src
Src
Dst
Dst
Collection of Sample SYNC
Collection of Sample NO-SYNC
Seq.Nr. + Transm. TSArrival timeRetransm. TSArrival time
The post-processing tool calculates:For each direction : Mean ipdv value, Values exceeding +/- LIM, Standard Deviation (total)St. Dev. over subset {-LIM < x < LIM}, Distributions.The same is provided for Time intervals into which the measurement can be subdivided.
Simulator
Measurement with simulated trafficresults
Conditions: Duration : 24 hours
Emission Interval (average) : 1 sec H-W time uncert. and vice versa : 100 µs
Delay : 100 ms to 500 ms Delay variation for traffic variation in 4 hours : + or - 300 ms
Packet Loss Ratio : 5 E-3 Skew of not sync. host : 50 ppm
Drift of not sync. host in 6 hours : 50 ppm
RESULTS: W-E Sync E-Wsync. W-Enot-sync E-Wnot-sync
Mean ipdv 1.159 µs 1.926 µs 55.578 µs 52.572 µsvalues <-100ms 20158 20217 20157 20200values > 100ms 20193 20236 20170 20222S.D total 160.3701 ms 160.6403 ms 160.3767 ms 160.5712 ms
Measurements of ipdvlayout
- Independent Measurements were performed at the same time in the two directions
- Data were recorded and then post-processed
Host in Torino (Italy)
Src
Dst
Host in Tokio (Japan)
Dst
SrcIP
20 hops
24 hops
GPSGPS
NO SYNC. SYNC.
Two measuremests of 24 hours:- The first with Synchronized Clocks (from 3pm of 3rd March to 3pm of 4th March)-The second with NOT Synchronized clocks (from 3pm of 5th March to 3pm of 6th March)
Measurement with Synchronized Clocks (1)
Type of Packets used : UDPEmission Interval : pseudo-random Poisson process
T average = 0,764 sec W-E 0.764 sec E-WTmin = 14.5 ms W-E 17 ms E-WT max = 8.999 sec W-E 8.989 ms E-W
Duration : 24h 00m 11s W-E 24h 00m 17s E-WNumber of generated packets : 113045 W-E 113079 E-W
Detected conditionsPackets Lost : 794 (0.7%) W-E 4609 (4.08%) E-WOut of sequence : 2 W-E 3 E-WReplicated packets : 0 W-E 0 E-WMean One-way-Delay : 279.9 ms W-E 384.6 ms E-W
Measurement with Synchronized Clocks (2)
RESULTS East to West (over 24 hours) :
Mean ipdv value : 0.27 [µs]Nr. of ipdv values < - 100 ms : 1421Nr. of ipdv values > 100 ms : 1429Nr. of ipdv values < - 1 s : 3Nr. of ipdv values > 1 s : 3Total Standard Deviation : 44.3 [ms]S.D. of subset {-100ms < x <100ms} 31.8 [ms]S.D. of subset {1s<x<1s} 43.5 [ms]
GPS equipment indicated a correction of about 77,7 µs / sec
Measurement with Synchronized Clocks (2)
RESULTS West to East (over 24 hours) :
Mean ipdv value : 0.02 [µs]Nr. of ipdv values < - 100 ms : 4961Nr. of ipdv values > 100 ms : 4861Nr. of ipdv values < - 1 s : 25Nr. of ipdv values > 1 s : 57Total Standard Deviation : 97.37 [ms]S.D. of subset {-100ms < x <100ms} 21.86 [ms]S.D. of subset {1s<x<1s} 91.53 [ms]
Measurement with Synchronized Clocks (3)
0
0.05
0.1
0.15
0.2
0.25
0.3
-0.2
50
-0.2
00
-0.1
50
-0.1
00
-0.0
50
0.00
0
0.05
0
0.10
0
0.15
0
0.20
0
0.25
0
E-W
W-E
sec
Distributions of ipdv values: SYNC.
Measurement with Synchronized Clocks (3)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
-0.2
50
-0.2
00
-0.1
50
-0.1
00
-0.0
50
0.00
0
0.05
0
0.10
0
0.15
0
0.20
0
0.25
0
E -W
W -E
Cumulative distributions :
sec
One way delay
0
200
400
600
800
1000
1200
1400
1 17 33 49 65 81 97 113
129
145
161
177
193
209
225
241
257
273
289
305
321
337
353
369
385
Packet num ber
One
way
del
ay [m
s]
E-W W-E
Measurement with Synchronized Clocks (3)
A sample of Delays :
Measurement with NOT Synchronized Clocks (1)
Type of Packets used : UDPEmission Interval : pseudo-random Poisson process
T average = 0.764 sec W-E 0.764 sec E-WTmin = 10 ms W-E 18 ms E-WT max = 9.36 ms W-E 8.99 ms E-W
Duration : 23h 48m 50s W-E 24h 03m 48s E-WNumber of generated packets : 112131 W-E 113361 E-W
Detected conditionsPackets Lost : 1154 (1.03%) W-E 4075 (3.59%) E-WOut of sequence : 3 W-E 10 E-WReplicated packets : 0 W-E 0 E-WMean One-way-Delay : Not measurable in these conditions
Measurement with NOT Synchronized Clocks (2)
RESULTS West to East (over 24 hours) :
Mean ipdv value : 56.64 [µs]Nr. of ipdv values < - 100 ms : 4730Nr. of ipdv values > 100 ms : 4656Nr. of ipdv values < - 1 s : 18Nr. of ipdv values > 1 s : 26Total Standard Deviation : 87.35 [ms]S.D. of subset {-100ms < x <100ms} 20.75 [ms]S.D. of subset {1s<x<1s} 83.78 [ms]
Being the mean emmission interval of 0.764 secthis means a clock skew of 56.64/0.764 = 74.136 ppm
Measurement with NOT Synchronized Clocks (2)
RESULTS East to West (over 24 hours) :
Mean ipdv value : -59.36 [µs]Nr. of ipdv values < - 100 ms : 1280Nr. of ipdv values > 100 ms : 1286Nr. of ipdv values < - 1 s : 1Nr. of ipdv values > 1 s : 2Total Standard Deviation : 40.23 [ms]S.D. of subset {-100ms < x <100ms} 29.98 [ms]S.D. of subset {1s<x<1s} 39.81 [ms]
Being the mean emmission interval of 0.764 secthis means a clock skew of -59.36 / 0.764 = -77.69 ppm
Measurement with NOT Synchronized Clocks (3)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
-0.2
50
-0.2
00
-0.1
50
-0.1
00
-0.0
50
0.00
0
0.05
0
0.10
0
0.15
0
0.20
0
0.25
0
E-W
W-E
Distributions of ipdv values:
sec
NOT SYNC.
Measurement with NOT Synchronized Clocks (3)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
-0.2
50
-0.2
00
-0.1
50
-0.1
00
-0.0
50
0.00
0
0.05
0
0.10
0
0.15
0
0.20
0
0.25
0
E-W ns
W-E ns
Cumulative distributions :
sec
Measurement with NOT Synchronized Clocks (3)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
1
207
410
616
825
1035
1248
1472
1677
1889
2099
2310
2529
2748
2964
3173
3392
3601
3808
E-W
W-E
A sample of “pseudo”Delays :
sec
Further points for study (1)
1) - Measurements with looped packets (measurements of Round trip delay at thesame time)
2) - Detection and characterization of bursts produced by the network3) - Measurements with separate pairs of thest pachets instead of using a
continuous stream4) - Possibility of measurements interleaving with user traffic.5) - Unique tool for measuring:
Round Trip Delayipdv on both directionsPacket Loss RatioOut of sequence RatioReplicated Packets Ratio
Some points that could be further explored are:
Further points for study (2)Example
Detection of bursts generated by network
Emission Times
Arrival Times
ipdv